The transporting of refrigerant fluids in vapor compression circuits, such as those employed more particularly in motor vehicle air conditioning, necessitates the use of materials having a very specific set of mechanical, thermal, and chemical properties.
The elements in these circuits (and especially the tubing) are required in particular:                to be impervious to the fluids being transported (and in particular to fluorocarbon refrigerant compounds), and also to water and to oxygen;        to exhibit chemical resistance to the fluids being transported, and also to water and to oxygen, in order to prevent excessive degradation over the long term;        to exhibit sufficient mechanical resistance but also sufficient flexibility (especially in motor vehicle air conditioning, where the under-hood assembly and congestion constraints dictate folding of the tubing);        to exhibit satisfactory heat resistance, owing to the fact that the fluids being transported may be at a high temperature, and that the temperature of the environment may also be high (especially in motor vehicle air conditioning, since the components in question may be positioned close to the engine).        
The above constraints are particularly difficult to observe with certain refrigerant fluids. For example, the fluid R-1234yf (2,3,3,3-tetrafluoropropene), which can be used more particularly in motor vehicle air conditioning, is more chemically reactive than the fluids of the preceding generation, such as R-134a (1,1,1,2-tetrafluoroethane). The chemical resistance and the barrier properties of the elements in the vapor compression circuits are therefore even more critical. The difficulty of observing the constraints above is exacerbated by the fact that certain lubricants, when used in a mixture with refrigerant compounds (such as polyalkylene glycols or PAGs), also have a tendency to be aggressive with respect to these circuit elements.
At the present time, the tubing for transporting refrigerant fluids in motor vehicle air conditioning comprises rigid metallic portions (generally made of aluminum) and flexible portions made of multilayer tubes. Some of these multilayer tubes are known as veneer tubes; they comprise in succession, from the outside to the inside, a first layer of rubber-type elastomer, a reinforcing braid, a second layer of rubber-type elastomer, and an inner layer based on polyamide.
In the versions available commercially, the inner layer based on polyamide (or PA) may be, for example, a formulated PA 6 (polycaprolactam) (with or without plasticizer, with or without impact modifier, with or without stabilizer), a formulated PA 6/6.6 copolyamide (with or without plasticizer, with or without impact modifier, with or without stabilizer), or else an alloy PA or PA 6.6 (polyhexamethylene adipamide) with polyolefins and functionalized polyolefins (product sold under the brand name Orgalloy® by Arkema).
Furthermore, document US 2007/0048475 describes a variant of these multilayer tubes, wherein the inner layer is based on PA 9.T (polymer of 1,9-nonanediamine or 2-methyl-1,8-ottanediamine and terephthalic acid), optionally blended with another polyamide resin.
Document US 2012/0279605 describes another variant of the multilayer tubes above, wherein the inner layer is a composition comprising a polyamide, an impact modifier, a phenolic antioxidant, a plasticizer, and a copper-based heat stabilizer. The polymer may be selected from a list of polymers including, in particular, PA 6/6.T (copolymer containing PA 6 segments and PA 6.T segments, in other words a polymer of hexamethylenediamine and terephthalic acid). The preferred polyamide, however, is PA 6.
Document US 2011/0136957 describes a resin for the inner layer of a vapor compression circuit, the resin being said to be chemically resistant toward R-1234yf. The resin is selected from a long list of polymers, including polyamides and more particularly semi-aromatic polyamides. In the examples, the resins used are based on PTFE (polytetrafluoroethylene) of polyimide.
Document US 2011/0183095 describes tubes for transporting heat transfer fluids in motor vehicle air conditioning, comprising an inner layer based on PA 6.10 (polyhexamethylene sebacamide) and an outer layer based on a polyamide such as a polyphthalamide. The polyphthalamide exemplified is PA 9.T.
Document US 2011/0239674 describes joints for a vapor compression circuit, made from the same materials. Document US 2012/0018995 also describes assemblies of tubes and joints that are made from the same materials. Documents WO 2010/061289 and US 2011/0277492 further describe vapor compression circuit elements made from these same materials.
Document US 2011/0155359 relates to tubes for transporting R-1234yf that are made from a resin based on semi-aromatic polyamide. The semi-aromatic copolyamide used in the examples is PA 6.6/6.T (copolymer of PA 6.6 and PA 6.T).
In a more general context, document EP 1505099 describes compositions intended for replacement of rubbers or metals, or for use as materials of electrical cables, or for the manufacture of tanks, hoses, and containers. These compositions comprise copolyamides of general formula PA X/Y.Ar (Ar meaning aromatic), including PA 11/10.T, which results from the condensation of aminoundecanoic acid, 1,10-decanediamine, and terephthalic acid.
Document WO 2006/037615 describes compositions based on semicrystallized polyamides for the manufacture of flexible tubes for transporting different fluids such as fuels (gasoline or diesel), hydraulic braking fluid, and others. PA 6.10, PA 6.12, and PA 10.10 are given as examples.
Document EP 1717022 relates to multilayer tubes for various applications, and more particularly for the transport of fuel in vehicles, from the tank to the engine. These tubes comprise an intermediate polyamide layer, made for example of PA 6.10 (polyhexamethylene sebacamide) or PA 6.12 (polyhexamethylene dodecanamide).
Documents WO 2010/015785 and WO 2010/015786 describe compositions based on semi-aromatic copolyamide of general formula A/10.T for the manufacture of various articles, for instance consumer goods such as electrical, electronic, or automotive equipment, surgical hardware, packaging, or else sports articles.
Similar applications are envisaged in document WO 2011/015790, which relates to compositions based on copolyamide 11/10.T, 12/10.T, etc.
Document WO 2011/077032 aims to provide a semi-aromatic polyamide which can be used generally in the manufacture of a variety of articles, for instance electronic components intended for electrical or electronic equipment in the field of road transport, road traffic or rail traffic, in the aeronautical, audio-video, and videogames fields, and also in the industrial sector. This semi-aromatic polyamide may be, among others, of formula 10/10.T, 11/10.T, 12/10.T, etc.
Document US 2011/0123749 describes a copolyamide corresponding to the condensation of 1,6-hexanediamine, 1,10-decanediamine, terephthalic acid, and at least one further monomer selected from the group of dicarboxylic acids comprising from 8 to 18 carbon atoms, laurolactam, aminolauric acid, and combinations thereof. The intended uses are as containers and conduits in the automotive sector, as for example fuel, oil, coolant, or urea lines.
None of the above documents describes polymeric material having properties that are fully satisfactory for the transport of heat transfer fluid, especially in motor vehicle air conditioning, and especially when the heat transfer fluid is a hydrofluorocarbon compound (HFO) such as R-1234yf (mixed with a lubricating oil).
There is therefore a need for development of vapor compression circuit elements that display an effective compromise between their properties of imperviousness to the heat transfer fluid, to water, and to oxygen, their chemical resistance to the heat transfer fluid, to water, and to oxygen, their mechanical strength, their flexibility, and their heat resistance. This need is felt particularly in the area of motor vehicle air conditioning, especially when the heat transfer fluid is R-1234yf, admixed with a lubricant such as a polyalkylene glycol (PAG) or else polyol ester (POE) oil.